Did you spend time together at your home, or did you book accommodation during the 6 months? The main evidence you are wanting to include here are any receipts showing you in the same place at the same time, also docs proving your fiancé travelled to the US (flight itinerary, boarding passes, ESTA/visa approval, etc). Although for my second trip, I stayed with my fiancé so we had no accommodation bookings, only my flights and boarding passes.
Also, how long is your relationship (including the time before you met)? Another red flag may be short term relationship before meeting, they stay with you for 6 months and now you are petitioning for a visa.
The beginning of this process is the most overwhelming part. You/your fiancé will experience the same when it comes to AOS, and I'm sure other steps beyond that. But once things start to come together, it starts to make sense.
If it helps, here is the list of what was included in my original application:
1) Two Passport-Sized Photos (Petitioner, Beneficiary)
2) Form G-1145 for E-Notification of Application Acceptance (1 Page)
3) Application Fee of $535 via Cashier’s Check
4) Form I-129F, Petition for K-1 Fiancé(e) for Beneficiary (14 Pages)
5) Proof of Meeting Within the Last 2 Years:
• Confirmation of Approved ESTA for Beneficiary (5 Pages)
• Airline Tickets & Flight Itinerary [November 2023] (3 Pages)
• Airbnb Reservation [November 2023] (1 Page)
• Boarding Passes to and from the USA for Beneficiary [November 2023] (2 Pages)
• Picture Together [November 2023] (1 Page)
• Airline Tickets & Flight Itinerary [March 2024] (2 Pages)
• Boarding Passes to the USA for Beneficiary [March 2024] (1 Page)
6) Proof of Relationship:
• Communication Transcripts (10 Pages)
7) Fiancé Letter of Intent to Marry within 90 days, executed by Petitioner (1 Page)
8) Fiancée Letter of Intent to Marry within 90 days, executed by Beneficiary (1 Page)
9) Statement of Circumstances of Meeting (2 Pages)
10) Proof of United States Citizenship for Petitioner (2 Pages)
11) Passport Copy for Beneficiary (1 Page)
12) Form I-94, Travel History for Beneficiary (2 Pages)
It may help you to organise your thoughts if you start by creating your own list, and gradually working through it. There are also many YouTube videos and other resources that can help with understanding questions.
I am glad you are rethinking the agency. The money would be better spent elsewhere to contribute to this very long and expensive process...